1. Field of the Invention
The present invention relates to an autofocus device used in optical instruments such as microscopes.
2. Description of the Related Art
In recent years, the use of optical instruments such as microscopes that employ autofocus (hereinafter, referred to as “AF”) techniques has been wide-spread. Particularly, the follow-AF function is commonly included in microscopes, by which proper focus is continuously maintained in the position that has to be the in-focus position by using the AF technique in order to achieve a high efficiency in observation of samples.
When, for example, reactions to the dosage of reagent to cells are observed in the conventional method, the in-focus position that has been achieved sometimes gets shifted due to vibration, distortion, or the like of vessels caused by the dosage. Even in such cases, microscopes can continuously maintain focus on the in-focus position if they have the follow-AF function.
However, because of some of the characteristics of microscopes, there are some difficulties in having the AF techniques applied to them. One of the problems occurs when the position on which proper focus is being achieved by the AF and the position that the observer is actually paying attention to are different on the sample.
In particular, in the case of a microscope using an objective lens with a high numerical aperture to achieve a high magnification, the focus depth of the objective lens is small, on the order of sub microns, while even the thinnest cell adopted as a sample has a thickness of several micrometers. Accordingly, it often occurs that the in-focus position fixed by the AF function and the position of the observation target that the observer is paying attention to on the sample are not the same.
In order to cope with this, a device is provided, as a conventional technique, in which the in-focus position can be changed by applying an offset, which can be done by using the AF without cancelling the AF in the case when the focus is manually adjusted during the AF operation. (For example, Japanese Patent Application Publication No. 2001-296467) The concept of this offset operation in the AF function is explained in FIG. 1.
FIG. 1 shows the principle of the conventional AF. An optical image of a sample 143 mounted on a stage 144 is formed into an image by an image forming lens 141 via an objective lens 140, and is picked up by an AF sensor 142; thereby visual observation is allowed from an observation position X.
By moving the sample 143 together with the stage 144 upward and downward in this configuration, the in-focus image of the sample 143 is projected onto the AF sensor 142. When parfocality is achieved between the AF sensor 142 and the observation position X by performing the AF operation, an in-focus image of the sample 143 can be observed from the observation position X.
In this configuration, the AF sensor 142 is moved in the direction of “a1” in the figure, and an offset is applied to the parfocality between the observation position X and the AF sensor 142. Then, the light image of the sample 143 projected onto the AF sensor 142 is defocused.
Thus, in order to obtain a projection of the in-focus image on the AF sensor 142 by using the AF control, the stage 144 is moved in the direction of “a2” in the figure. At this moment, the observation position X has not moved, and thus an out-of-focus image of the lower side of the sample 143 is to be observed from the observation position X.
When the AF sensor 142 is moved in the direction of “b1”, the stage 144 is moved in the direction of “b2” in a similar operation as above. Then, an out-of-focus image of the upper side of the sample 143 is to be observed from the observation position X.
By utilizing the above phenomena, a so-called offset function of the AF is achieved in which it is possible to obtain an in-focus image of respective layers at different heights in the sample 143 of, for example, a multilayer configuration, by performing the AF control from arbitrarily changed positions.
Thereby, even when the position on the sample 143 on which proper focus is being achieved by the AF and the position to which the observer is actually paying attention are different from each other, proper focus can be achieved on the position to which attention is being given by the observer by applying an offset.
Japanese Patent Application Publication No. 2002-341234 discloses an AF device for a microscope in which the initial adjustment error between the active and passive AFs is reduced in the hybrid AF. According to the technique of Japanese Patent Application Publication No. 2002-341234, the adjustments of the active and passive AFs that were separately performed in conventional methods are performed in such a manner that after adjustment of the passive AF, the initial adjustment of the active AF is performed while monitoring the focusing state in the adjusted passive AF. Thereby, focal shifts between the active and passive AFs that are due to the lack of adjustment and mistakes in adjustment are reduced.